The long-term objective of this study is to understand mechanisms underlying Ca2+-mediated exocytosis during fast synaptic transmission. Chemical synapses are common sites for the manifestation of plastic changes thought to underlie some forms of learning, memory and developmental changes in the central nervous system. Moreover, synapses are also the targets of many neuroactive agents such as toxins and therapeutic drugs, and can become dysfunctional in certain types of neuropathology. A better understanding of synaptic function is therefore likely to be vital for advances in medical science. The mechanisms underlying exocytosis will be investigated in hair cells from the frog sacculus. These cells are ideally suited for the study of synaptic mechanisms, particularly the roles of Ca2+ and Ca2+-binding proteins (calbindin-D28k and synaptotagmin) in exocytosis. The specific aims are: (1) To test the hypothesis that the mobile Ca2+ buffer calbindin-D28k influences short-range Ca2+ signaling in frog saccular hair cells. (2) To test the hypothesis that increases in membrane capacitance induced by depolarization are due to the fusion of synaptic vesicles with the plasma membrane at active zones. This work will provide quantitative information about Ca2+ signaling and the release of neurotransmitter that should be applicable to other synapses in the nervous system.